Mobile radio systems have to be highly spectral efficient to allow high user capacities and high data rates. Multicarrier modulation realized by orthogonal frequency division multiplexing (OFDM) is well suited for high data rate applications in fading channels and has been chosen for several new standards like digital audio broadcasting (DAB) and broadband LAN standards such as, for example, HIPERLAN/2, IEEE 802.11 and Multimedia Mobile Access Communications (MMAC) respectively.
Code-division multiple access (CDMA) is a multiplexing technique where a number of users simultaneously access a channel by modulating and spreading their information-bearing signals with pre-assigned signature sequences. Recently, the concept of combining OFDM signaling with CDMA to provide a multicarrier CDMA scheme has attracted significant research interests. One major advantage of the multicarrier CDMA scheme is that symbol rate can be lowered for each subcarrier. In other words, longer symbol duration enables easier quasi-synchronizing of transmissions in communication systems having the multicarrier CDMA scheme.
The multicarrier CDMA scheme is categorized into two main groups. One group spreads an original data stream in the time domain such as multicarrier Direct Sequence (DS)-CDMA and multi-tone (MT) CDMA. The latter group performs the spreading operation in the frequency domain, transmitting all the chips of a spread symbol at the same time, but in different orthogonal sub-channels. Communication systems in the latter group are commonly referred to as MC-CDMA communication systems. FIG. 1 is a general block diagram of a prior art MC-CDMA communication system 10 having a transmitter 12 and a receiver 14. A channel 16 provides a communication medium for signals to be transmitted from the transmitter 12 to the receiver 14, which processes received signal input 18 corresponding to transmitted signals.
Various detectors have been proposed and analyzed for MC-CDMA communication systems. For example, in “Performance of CDMA/OFDM for mobile communication systems” [2nd IEEE International Conference on Universal Personal Communications (ICUPC), pp. 975-979, 1993] K. Fazel describes an optimal Maximum Likelihood Detector (MLD). However, as complexity grows exponentially with the number of users, the MLD can be used only for a small number of interfering users. This difficulty led to the consideration of sub-optimal but simpler detectors. Such simpler detectors include Equal Gain Combining (EGC) detectors, Maximum Ratio Combining (MRC) detectors, Minimum Mean Square Error (MMSE) combining detectors, Multi-User Detectors (MUD) for interference cancellation, Orthogonality Restoring Combining (ORC) detectors, and ORC with Threshold (TORC) detectors that is also known as controlled equalization.
Of the above detectors, only an ORC detector can effectively eliminate multi-user interference and there is no error floor in the bit-error-rate (BER) performance. However, noise components are amplified at weaker subcarriers in a receiver having the ORC detector. The weaker a subcarrier, the higher the noise amplification of that subcarrier. To address this problem, TORC detectors are proposed to suppress excessive noise by setting a threshold for signal amplitudes. Components corresponding to weaker subcarriers that have amplitudes smaller than the threshold are discarded. Noise level is reduced with rising threshold in the TORC detector because fewer weak subcarriers are accepted. However, for TORC detectors, orthogonality of signals is destroyed and, consequently, interference results. As more subcarriers are discarded, the interference that is introduced increases. BER depends on noise level as well as the interference. The interference is especially responsible for error floor at high signal-to-noise ratios (SNR).
Considering both effects of excessive noise amplification and interference, an MMSE detector compromises between these two effects. When noise level is low (higher SNR), the MMSE detector works similarly to ORC detector to recover orthogonality amongst users. When noise level is high (lower SNR), the MMSE detector reduces the noise. In this way, the MMSE detector can suppress the excessive noise amplification without error floor in the high SNR region. However, system performance between the MMSE detector and the optimal MLD detector is still quite large.
Therefore, a need clearly exists for an MC-CDMA receiver that alleviates excessive noise amplification of weak subcarriers in general and interference in particular when MMSE or TORC detectors are used in the MC-CDMA receiver. Furthermore, such an MC-CDMA receiver should accommodate more users without substantially increasing complexity when compared with existing MC-CDMA receivers having MLD detectors.